Cut potatoes into 1cm thick slices. Cut slices into 1cm thick chips. Layer between sheets of paper towel to remove excess moisture. Stand for 10 minutes.
Heat oil in a wok or large deep frying pan over medium-high heat. Deep-fry potato, in batches, until potato rises to the surface and turns opaque. Using a slotted spoon, transfer chips to a tray lined with paper towel. Cool for 15 minutes.
Reheat oil. Deep-fry chips, in batches, for a further 3 to 4 minutes or until golden. Drain on paper towel. Transfer to a bowl. Add salt and rosemary. Toss to coat. Serve.